The letter was addressed to Donald Trump Jr. and sent to his mother-in-law's apartment on East 54th Street in Manhattan.
This was the scene outside their Sutton Place apartment early Monday afternoon:
There, Trump Jr.'s wife, Vanessa, opened the letter just after 10 a.m. She and two other people were decontaminated by firefighters at the scene. Vanessa Trump was the only person taken to the hospital for evaluation.
Everyone exposed to the powder appears to be fine, officials said.
"The substance was deemed to be non-hazardous and is being transported to a lab in New York City for further analysis," NYPD spokesman Peter Donald said.
PHOTOS: Scene of Donald Trump's apartment in NYC
Police and Secret Service agents are at the scene investigating.
This is a breaking news story. Keep checking abc7NY.com for the latest information.
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